Polymer solar cell and method of manufacturing the same

ABSTRACT

Disclosed herein are a polymer solar cell, comprising a substrate, a first electrode, a hole injection layer, a photoactive layer, and a second electrode, characterized in that an electron-accepting layer is formed between the photoactive layer and the second electrode, and a method of manufacturing the polymer solar cell. The polymer solar cell comprises an electron-accepting layer between the photoactive layer and the second electrode, thereby assuring excellent power conversion efficiency. Furthermore, high power conversion efficiency can also be attained in a low-temperature thermal annealing process.

CROSS-REFERENCE TO RELATED PATENT APPLICATION

This non-provisional application claims priority under U.S.C. §119(a) toKorean Patent Application No. 2007-0001367, filed on Jan. 5, 2007, theentire contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a polymer solar cell and a method ofmanufacturing the same, and more specifically, to a polymer solar cellcomprising an electron-accepting layer between a photoactive layer and asecond electrode, capable of realizing increased power conversionefficiency. In particular, high power conversion efficiency can beattained even in a low-temperature thermal annealing process.

2. Description of the Related Art

In general, a solar cell is a photovoltaic device used for theconversion of solar light into electrical energy. A solar cell is usablewithout limitation, is environmentally friendly, unlike other energysources, and, is thus expected to become an increasingly importantenergy source over time.

Conventionally, a silicon solar cell made of monocrystalline orpolycrystalline silicon has been mainly utilized. However, the siliconsolar cell suffers from disadvantages because it has a highmanufacturing cost and cannot be applied to a flexible substrate. As analternative to the silicon solar cell, thorough research into polymersolar cells is currently ongoing.

The polymer solar cell may be manufactured through spin coating, ink-jetprinting, roll coating, or doctor blading, and therefore themanufacturing process is simple resulting in a low manufacturing cost.Further, the use of a polymer solar cell is advantageous because a largearea may be coated, a thin film may be formed even at low temperaturesand, almost any kind of substrate, including a glass substrate and aplastic substrate, may be used.

In addition, solar cells having various shapes may be manufactured, suchas curved or spherical plastic molded products, which may also be bentor folded so that they are easily portable. When making use of the aboveadvantages, a solar cell may be manufactured that can easily be attachedto people's clothes, bags, or be mounted to portable electrical orelectronic products. In addition, when a polymer blend film, having hightransparency to light, is attached to the glass windows of buildings orthe glass windows of automobiles, it can generate power whilesimultaneously allowing a person to see through the window.Consequently, polymer solar cells have a broader range of applicationthan opaque silicon solar cells.

Although the polymer solar cell possesses the above advantages, it isunsuitable for practical use because the power conversion efficiencythereof is low and the lifetime thereof is short. That is, by the end ofthe 1990s the power conversion efficiency of the polymer solar cell wasonly about 1%. However, since the year 2000, the performance of the cellhas begun to greatly increase through improvements in the structuralmorphology of the polymer blend. Presently, in the case where the powerconversion efficiency of the polymer solar cell is measured under solarlight conditions of AM 1.5 global 100 mW/cm², a unit device having asmall area (0.1 cm² or less) has power conversion efficiency of about 4to about 5%, and a device having an area of 1 cm² has power conversionefficiency of about 3% ((M. A. Green, K. Emery, D. L. King, Y. Hishikawaand W. Warta, Prog. Photovolt. Res. Appl. 14, 455-461(2006)).

Typically, a polymer solar cell comprises a first electrode, a secondelectrode, and a thin film layer composed of a conjugated polymer or aconductive polymer having semiconductor properties and, an electronacceptor between the first electrode and the second electrode.

An example of a polymer useful in a polymer solar cell, is a conductivepolymer such as polythiophene or p-phenylene vinylene (“PPV”)derivatives, that function as an electron donor. When the conductivepolymer absorbs light having a wavelength not less than an energy bandgap, it is excited to an exciton. The exciton binding energy of theconductive polymer typically ranges from 0.1 to 1.0 eV, which isconsiderably greater than thermal energy (about 0.025 eV) at roomtemperature. Accordingly, since the conductive polymer has a lowprobability of being separated into free electrons and complementarypositively charged holes, solar cells using a thin film composedexclusively of the conductive polymer have very low power conversionefficiencies of about 0.1% or less.

In order to increase the free electron production efficiency of a singlefilm comprising the conductive polymer, the use of a double filmcomposed of a conductive polymer and an electron acceptor has beenproposed. However, the exciton diffusion length in the polymersemiconductor, is of about 3 to about 10 nanometers (nm). Thus, in thethin double layer comprised of the conductive polymer and an electronacceptor, free charges are produced only in the narrow regioncorresponding to the heterojunction interface, which is about 3 to about10 nm thick, in the total thin film layer which is about 100 nm thick.Consequently, the charge production efficiency is still low.

With the goal of increasing the heterojunction interface between theconductive polymer and the electron acceptor, research is beingconducted into polymer solar cells that include a blend layer ofconductive polymer and an electron acceptor. As such, the heterojunctioninterface between the conductive polymer and the electron acceptor isdistributed over the entire internal portion of the thin film, and thusthe production of free charges may be effectively realized throughoutthe entire thin film layer. For example, power conversion efficiency ofabout 3.5% has been reported using a blend film of “P3HT”(poly(3-hexylthiophene)) and “PCBM” ([6,6]-phenyl-C₆₁ butyric acidmethyl ester) and a thin LiF buffer layer at the junction interface withan Al electrode. [F. Padinger, R. S. Rittberger, N. S. Sariciftci, Adv.Func. Mater., 13, 85(2003)] However, such a polymer solar cell still haslower power conversion efficiency than other thin-film solar cells, andextensive effort is required to further increase the power conversionefficiency of polymer solar cells.

In the polymer solar cell, various attempts have been made to increasethe crystallinity of the polymer through thermal annealing at about 120to about 160° C., in order to improve the low charge mobility of thepolymer due to the disordered structure of the polymer nanocomposite.

In the case where the solar cell is manufactured on a plastic substrate,it is difficult to perform thermal annealing due to problems related tothe thermal deformation of the plastic. Therefore, decreasing thethermal annealing temperature of the polymer blend is considered to bean important step in the development of plastic solar cells.

SUMMARY OF THE INVENTION

In one embodiment, the present invention provides a polymer solar cell,which comprises an electron-accepting layer between a photoactive layerand a second electrode, thereby assuring high power conversionefficiency even upon low-temperature thermal annealing.

In another embodiment, the present invention provides a method ofmanufacturing a polymer solar cell, which can prevent the deformation ofa substrate upon thermal annealing, thus making it possible tomanufacture a flexible solar cell with higher power conversionefficiency.

In accordance with one aspect of the present invention, there isprovided a polymer solar cell, comprising a substrate, a first electrodepositioned on the substrate comprising a conductive layer, a holeinjection layer positioned on the first electrode, a photoactive layerpositioned on the hole injection layer, an electron-accepting layerpositioned on the photoactive layer, and a second electrode.

In accordance with another aspect of the present invention, theelectron-accepting layer of the polymer solar cell may comprise aC₆₀-C₇₀ fullerene derivative, and preferably comprises PCBM([6,6]-phenyl-C₆₁ butyric acid methyl ester).

In accordance with yet another aspect of the present invention, there isprovided a method of manufacturing a polymer solar cell, comprising asubstrate, a first electrode, a hole injection layer, a photoactivelayer, and a second electrode, the method comprising forming anelectron-accepting layer between the photoactive layer and the secondelectrode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows an exemplary embodiment of a cross-section ofthe polymer solar cell, according to the present invention;

FIG. 2 is an exemplary embodiment that schematically shows across-section of the sample device for measuring the charge mobility ofa P3HT:PCBM blend layer;

FIGS. 3A and 3B are graphs that show the hole mobility of the P3HT:PCBMblend layer, measured at various temperatures (about 100 to about 440Kelvin (K)) and under various electric fields;

FIG. 4 is a graph comparing the photocurrent-photovoltage of aconventional solar cell having no PCBM film, manufactured throughapplication of the P3HT:PCBM blend layer and then thermal annealing at90° C., and the solar cell further including a PCBM film;

FIG. 5 is a graph comparing the photocurrent-photovoltage of aconventional solar cell manufactured through application of theP3HT:PCBM blend layer, deposition of the LiF/Al electrode, and thenthermal annealing at 90° C., and a solar cell manufactured throughdeposition of the PCBM film and the LiF/Al electrode and then thermalannealing at 90° C.; and

FIG. 6 is a graph comparing the photocurrent-photovoltage of aconventional solar cell having no PCBM film, manufactured throughapplication of the P3HT:PCBM blend layer and then thermal annealing at150° C., and the solar cell further including a PCBM film.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, a detailed description of the exemplary embodiments of thepresent invention will be provided, with reference to the accompanyingdrawings.

According to one embodiment of the present invention, the solar cellcomprises a substrate, a first electrode, a hole injection layer, aphotoactive layer, an electron-accepting layer, and a second electrode.

In another embodiment, the polymer solar cell includes theelectron-accepting layer between the photoactive layer and the secondelectrode, thereby increasing the interfacial properties of thephotoactive layer and the second electrode. Hence, high levels of powerconversion efficiency can be attained even in a low-temperature thermalannealing process.

FIG. 1 is an exemplary embodiment of the present invention illustratinga sectional view of the structure of the polymer solar cell.

As shown in FIG. 1, the polymer solar cell of the invention comprises asubstrate 10, a first electrode 20 comprising a conductive layerpositioned on the substrate 10, a hole injection layer 30 positioned onthe first electrode 20, a photoactive layer 40 positioned on the holeinjection layer 30, an electron-accepting layer 60 positioned on thephotoactive layer 40, and a second electrode 50.

In yet another embodiment, the polymer solar cell of the presentinvention is characterized in that the electron-accepting layer 60 isintercalated between the photoactive layer 40 and the second electrode50.

The electron-accepting layer 60 is preferably composed of materialhaving a good ability to capture electrons. Examples ofelectron-accepting layers include C₆₀-C₇₀ fullerene derivatives, carbonnanotubes, carbon nanotube derivatives, and the like. Particularlyuseful is PCBM.

Without being limited by theory, the principle governing the high levelsof power conversion efficiency attained by the polymer solar cell of thepresent invention, even at low annealing temperatures, is as follows.

In a typical polymer solar cell including only a photoactive layer(P3HT:PCBM blend layer), thermal annealing is performed at about 150° C.(420 Kelvin (“K”)) to increase the charge mobility of the photoactivelayer. Through such thermal annealing, PCBM molecules diffuse in theP3HT:PCBM blend layer, resulting in phase separation, in which the PCBMmolecules are agglomerated and, the production of PCBM clusters and P3HTcrystallites occur. When the PCBM clusters are produced, the electronmobility is increased. Furthermore, the hole mobility is also increasedin the P3HT crystallite.

According to exemplary embodiments of the present invention, as shown inFIGS. 3A and 3B, an increase in the charge mobility of the P3HT:PCBMblend layer through thermal annealing can be observed in the temperaturerange of about 340 to about 360 K, whereas at higher temperatures, thecharge mobility does not increase any further even though thetemperature is increased. Conversely, in the typical polymer solar cell,when thermal annealing is performed at temperatures lower than 150° C.(420 K), the efficiency is remarkably decreased.

A sufficient increase in the charge mobility of the P3HT:PCBM blendlayer is achieved through thermal annealing at about 340 to about 360 Kfor about 1 hour. Nevertheless, the increase in the power conversionefficiency requires thermal annealing at higher temperatures (about 150°C. (420 K)). This is because the thermal annealing at highertemperatures changes the interfacial properties of the P3HT:PCBM blendlayer and the electrode. That is, when the thermal annealing temperatureis increased, it is expected that the interface between the P3HT:PCBMblend layer and the electrode contains more PCBM molecules due to thediffusion of PCBM molecules having a low molecular weight to thesurface. Consequently, the interfacial properties between the P3HT:PCBMblend layer and the electrode affect the power conversion efficiency.

Without being limited by theory, the polymer solar cell of the presentinvention includes a thin PCBM film between the P3HT:PCBM blend layerand the second electrode. Therefore, even though thermal annealing isperformed at about 90° C., which is a considerably lower temperaturethan conventional thermal annealing, high power conversion efficiencycan be attained.

In one embodiment, the PCBM film preferably has a thickness of about 0.1to about 10 nm.

In another embodiment, the substrate of the polymer solar cell, is notparticularly limited as long as it is transparent, and may be atransparent inorganic substrate, such as quartz and glass, or atransparent plastic substrate selected from the group consisting ofpolyethylene terephthalate (“PET”), polyethylene naphthalate (“PEN”),polycarbonate (“PC”), polystyrene (“PS”), polypropylene (“PP”),polyimide (“PI”), polyethersulfonate (“PES”), polyoxymethylene (“POM”),AS resin, ABS resin and a combination comprising at least one of theforegoing.

In yet another embodiment, the substrate 10 preferably has a wavelengthtransmittance of greater than or equal to 70%, and more preferablygreater than or equal to 80% in the visible light range of about 400 toabout 750 nm.

In one aspect of the present invention, the first electrode 20 acts as apath for light that passes through the substrate 10 and reaches thephotoactive layer 40. Thus, the first electrode 20 preferably comprisesa highly transparent material. More preferably, the first electrode 20is comprised of a conductive material having a high work function ofgreater than or equal to 4.5 eV and, low resistance. Examples ofconductive materials useful for the first electrode 20 include, but arenot limited to, indium tin oxide (“ITO”), gold, silver, fluorine-dopedtin oxide (“FTO”), ZnO—Ga₂O₃, ZnO—Al₂O₃, and SnO₂—Sb₂O₃.

The holes separated in the photoactive layer 40 reach the firstelectrode 20 through the hole injection layer 30. The first electrode 20may be deposited on the substrate 10 through thermal evaporation, e-beamevaporation, RF or magnetron sputtering, chemical deposition, or methodssimilar thereto.

In another aspect of the present invention, the hole injection layer 30is formed of a conductive polymer. Examples of suitable conductivepolymers for the hole injection layer include, one or more selected fromthe group consisting of “PEDOT” (poly(3,4-ethylenedioxythiophene), “PSS”(poly(styrenesulfonate)), polyaniline, phthalocyanine, pentacene,polydiphenylacetylene, poly(t-butyl)diphenylacetylene,poly(trifluoromethyl)diphenylacetylene, Cu-PC (copper phthalocyanine),poly(bistrifluoromethyl)acetylene, polybis(t-butyldiphenyl)acetylene,poly(trimethylsilyl)diphenylacetylene, poly(carbazole)diphenylacetylene,polydiacetylene, polyphenylacetylene, polypyridine acetylene,polymethoxyphenylacetylene, polymethylphenylacetylene,poly(t-butyl)phenylacetylene, polynitrophenylacetylene,poly(trifluoromethyl)phenylacetylene,poly(trimethylsilyl)phenylacetylene, and derivatives thereof, and acombination comprising at least one of the foregoing polymers.Preferably, a mixture of PEDOT-PSS is used.

The conductive polymer material used for the hole injection layer may beapplied to a thickness of about 5 to about 2000 nm on the firstelectrode using a typical coating process, for example, spraying, spincoating, dipping, printing, doctor blading, or sputtering, or throughelectrophoresis.

In yet another aspect the present invention, the photoactive layer 40comprises a blend layer of a conductive polymer and, an electronacceptor, including a p-type conductive polymer material havingπ-electrons as an electron donor, and fullerene or derivatives thereof,as an electron acceptor.

Examples of suitable conductive polymers used for the photoactive blendlayer include, one or more selected from the group consisting of P3HT(poly(3-hexylthiophene), polysiloxane carbazole, polyaniline,polyethylene oxide, poly(1-methoxy-4-(O-disperse Red1)-2,5-phenylene-vinylene, polyindole, polycarbazole, polypyridiazine,polyisothianaphthalene, polyphenylene sulfide, polyvinylpyridine,polythiophene, polyfluorene, polypyridine, and derivatives thereof, anda combination comprising at least one of the foregoing conductivepolymers.

Examples of the electron acceptor used for the photoactive blend layerinclude, but are not limited to, fullerene or derivatives thereof,nanocrystals such as CdSe, carbon nanotubes, nanorods, nanowires, or thelike, or a combination comprising at least one of the foregoing electronacceptors.

In one embodiment, the photoactive layer is preferably composed of amixture of the electron donor, such as P3HT, and the electron acceptor,such as the fullerene derivative, for example, PCBM ([6,6]-phenyl-C₆₁butyric acid methyl ester), which are mixed at a weight ratio of about1:0.1 to about 1:2.

A photoactive layer of about 5 to about 2000 nm in thickness may also beapplied on the hole injection layer using a typical coating process, forexample, spraying, spin coating, dipping, printing, doctor blading, orsputtering, or through electrophoresis.

The electrons generated at the heterojunction interface of thephotoactive layer 40 reach the second electrode 50 through theelectron-accepting layer 60.

In another embodiment of the present invention, the second electrode 50comprises material having a low work function. Suitable examples of suchmaterial are, metals, such as magnesium, calcium, sodium, potassium,titanium, indium, yttrium, lithium, aluminum, silver, tin, lead, or thelike, or a combination comprising at least one of the foregoing metals.The second electrode may be deposited on the electron-accepting layerusing the same process used for the first electrode.

The second electrode 50 may have a multilayer structure obtained byforming a LiF or LiO₂ buffer layer on the electron-accepting layer andthen depositing the above electrode material on the buffer layer.

In yet another embodiment, the present invention provides a method ofmanufacturing a polymer solar cell comprising, forming a hole injectionlayer on a substrate comprising a conductive layer, forming aphotoactive layer on the hole injection layer, performing thermalannealing, forming an electron-accepting layer on the photoactive layer,and forming a second electrode on the electron-accepting layer.

In one embodiment, the thermal annealing is preferably performed atabout 80 to about 110° C.

According to the present invention, the method of manufacturing thesolar cell is not particularly limited, and any method may be usedwithout limitation as long as it is typically known in the art.

Herein, exemplary embodiments of the present invention will be describedin more detail with reference to the following examples. However, theseexamples are merely set forth to illustrate the invention, and thus arenot to be construed as limiting the scope of the present invention.

EXAMPLES Example 1 Measurement of Charge Mobility of P3HT:PCBM BlendLayer

(1) Fabrication of Measurement Device

PEDOT:PSS [Baytron P AI 4083poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate)] was applied ondry ITO glass through spin coating at 4000 rpm for 30 seconds (sec.).The coated device was dried in a vacuum oven at 120° C. for 30 minutes(min) or longer. A P3HT:PCBM blend solution was applied onto thesubstrate coated with the PEDOT:PSS film via spin coating at 1000 rpmfor 30 sec.

The P3HT:PCBM blend solution was prepared as follows. P3HT and PCBM wereseparately dissolved to 4 weight percent (wt %) in a chlorobenzenesolvent and then dispersed on a hot plate at 50° C. or higher for 30 minor longer using stirring magnets. The P3HT solution and the PCBMsolution were filtered using a 0.3 micrometer (μm) PVDF filter, and a0.5 μm PVDF filter, respectively. By adjusting the ratio of theindividual P3HT and PCBM solutions, P3HT:PCBM blend solutions havingvarious concentrations were prepared.

The P3HT:PCBM blend film was formed and then dried at room temperaturefor 1 hour or longer in a nitrogen atmosphere to remove the solvent.Following the drying process, LiF and Al were deposited on the P3HT:PCBMblend film through vacuum thermal evaporation, thus manufacturing asolar cell. The structure of the solar cell is shown in FIG. 2. Thedevice for measuring the “TOF-PC” (time-of-flight photocurrent) of thefilm, was coated so that the thickness of the film was about 1 μm ormore. For this, the concentration of the P3HT:PCBM blend was increasedto 10 wt %, and the blend was applied via spin coating at a rate of 800rpm. The thickness of the P3HT:PCBM blend film was measured to be 1.1 μmusing an Alpha-step 500.

(2) Measurement of Charge Mobility

The polymer blend solar cell was mounted to a cryostat in order tofacilitate measurements in the temperature range from about 100 K toabout 440 K. Using a KEITHLEY 236 DC source meter, current-voltage(“I-V”) was measured both in the dark (dark I-V) and upon lightirradiation, and the photocurrent spectrum was determined. The lightsource used for the experiment was a Muller Xenon lamp 300 W (Watts).When the photocurrent was measured, the intensity of light wascontrolled to be AM 1.5 (about 100 mW/cm²), which equals general solarlight intensity using a Neutral density filter.

Using the TOF-PC method, the charge mobility of the P3HT:PCBM blend wasmeasured. Voltage was applied to the device for measuring TOF-PC usingan E3614A power meter, available from Agilent. Further, using a GL-3300nitrogen laser available from “PTI” (Photon Technology International),laser pulses at 337 nm were applied through the ITO electrode in orderto generate a photocurrent. The photocurrent was then measured using adigital oscillator (Tektronix TDS 5054B). The charge transfer timeτ_(tr) was determined based upon the photocurrent measurement, and thecharge mobility was then calculated using the following equation:μ=d ²/(Vτ _(tr))  [Equation 1]

In Equation 1, d is the thickness of the device, V is the appliedvoltage, and τ_(tr) is the charge transfer time.

(3) Analysis of Results

FIGS. 3A and 3B show the charge mobility of the P3HT:PCBM blend layer,measured at various temperatures and under various electric fields. Theelectron mobility (FIG. 3B) was greater than the hole mobility (FIG. 3A)throughout the entire temperature range. At room temperature, the holemobility was measured to be 5.73×10⁻⁵ cm²/Vs, and the electron mobilitywas measured to be 1.64×10⁻⁴ cm²/Vs. When comparing the above resultsP3HT:PCBM/PCBM) with those of the P3HT:PCBM film, the hole mobility ofthe P3HT:PCBM blend layer was slightly decreased whereas the electronmobility was slightly increased. The reason for this difference isthought to be because the addition of the PCBM, which has high electronmobility, leads to the increased electron mobility of the P3HT:PCBMfilm, and causes the structure of the P3HT to be highly disordered, thusdecreasing the hole mobility. When investigating the effect oftemperature on charge mobility, it was observed that both the holemobility and the electron mobility have a tendency to increase dependingon the temperature. In particular, when the temperature was increasedfrom 340 K to 360 K, the hole mobility and the electron mobility weregreatly increased. Further, the hole mobility was observed tocontinuously increase at 360 K for about 1 hour at which point itreached an equilibrium state. The hole mobility at 360 K increased from4.27×10⁻⁵ cm²/Vs at 10 minutes, to 1.13×10⁻⁴ cm²/Vs at 32 minutes, andthen to 1.19×10⁻⁴ cm²/Vs at 52 minutes. At temperatures of 360 K orhigher, almost no increase in electron mobility or hole mobility wasobserved.

Example (2) Fabrication of a Polymer Solar Cell Comparative Example 1

A glass substrate coated with ITO was dipped in distilled water, inwhich a detergent was dissolved, and was then subjected to ultrasoniccleaning for 30 min. Subsequently, the glass substrate was dipped indistilled water, subjected to ultrasonic cleaning for 5 min, and thenthe washing was repeated twice more.

After the completion of the distilled water washing, the glass substratewas subjected to ultrasonic cleaning in the solvents isopropyl alcohol,acetone, and methanol, in that order, and was then dried. The glasssubstrate coated with ITO was subjected to plasma treatment for 5 minunder the conditions of 14 mtorr pressure and 50 W power, using nitrogenplasma in a plasma cleaner.

On the ITO transparent electrode, 1 ml of a PEDOT:PSS solution (BaytronP AL 4083, available from Bayer), mixed 1:1 in chlorobenzene, was spincoated at 4,000 rpm for 30 sec. This process formed a hole injectionlayer 200 nm thick, which was then dried in a vacuum oven at 120° C. for30 min.

Subsequently, P3HT and PCBM were dissolved in chlorobenzene at a weightratio of 1:0.8, and then spin coated at 1,000 rpm for 30 sec on top ofthe hole injection layer, thus forming a photoactive layer 1100 nmthick, which was then dried for 30 min in a nitrogen atmosphere. Assuch, the ratio of the P3HT and PCBM relative to the solvent was 2 wt %.

Thereafter, thermal annealing was performed at 90° C. for 30 min, a PCBMfilm was deposited to a thickness of 5 nm using a vacuum depositionmachine, and then LiF/Al was deposited as a second electrode, therebymanufacturing a solar cell.

Comparative Example 2

A solar cell was manufactured in the same manner as in Example 1, withthe exception that the thermal annealing was not performed after thephotoactive layer was formed, but was performed after the PCBM film andthe LiF/Al electrode were deposited.

Comparative Example 3

A solar cell was manufactured in the same manner as in Example 1, withthe exception that the thermal annealing was performed at 150° C.

Comparative Examples 4-6

Solar cells were manufactured in the same manner as in ComparativeExamples 1-3, with the exception that the PCBM film was not formedbetween the blend layer of conductive polymer and electron acceptor and,the second electrode.

The photovoltage and the photocurrent of the solar cells manufactured inthe comparative examples were measured. The results are shown in FIGS.4, 5 and 6. As such, as a light source, a Xenon lamp (01193, availablefrom Oriel) was used, and the solar conditions (AM 1.5) of the Xenonlamp were adjusted using a standard solar cell (Furnhofer InstituteSolare Energiesysteme, Certificate No. C-ISE369, Type of material:Mono-Si+KG filter). Short-circuit current (I_(sc)), open-circuit voltage(V_(oc)) and a fill factor (FF), calculated from the above measuredphotocurrent-photovoltage curve, were substituted into the followingequation, in order to calculate photovoltaic efficiency (η_(e)). Theresults are shown in Table 1 below.η_(e)=(V _(oc) ·I _(sc) ·FF)/(P _(inc))  [Equation 2]

In Equation 2, P_(inc) is 100 mw/cm² (1 sun).

TABLE 1 After Coating of P3HT:PCBM (1:0.8), Temp. and Power ConversionSequence of Thermal Annealing, and Whether V_(OC) I_(SC) J_(SC)Efficiency (%) Deposition of PCBM or Not (V) (μA) (mA/cm²) FF (AM 1.5G100 mW/cm²) C. Ex. 1: After Annealing at 90° C. for 30 min, 0.561 1457.41 0.42 1.73 PCBM(30 Å)/LiF/Al Deposition C. Ex. 2: After PCBM(30Å)/LiF/Al Deposition, 0.559 123 6.29 0.45 1.60 Annealing at 90° C. for30 min, C. Ex. 3: After Annealing at 150° C. for 30 min, 0.532 154 7.840.42 1.76 PCBM(30 Å)/LiF/Al Deposition C. Ex. 4: After Annealing at 90°C. for 30 min, 0.578 119 6.08 0.33 1.18 LiF/Al Deposition C. Ex. 5:After LiF/Al Deposition, Annealing at 0.574 118 6.03 0.47 1.63 90° C.for 30 min C. Ex. 6: After Annealing at 150° C. for 30 min, 0.555 1366.94 0.45 1.74 LiF/Al Deposition

As is apparent from Table 1, the properties of the device in which theP3HT:PCBM blend layer was applied and a PCBM layer 5 nm thick wasdeposited at the interface between the polymer-electron acceptor and theLiF/Al electrode, were superior to those of conventional solar cellswithout the PCBM layer. In particular, the conventional solar cell(Comparative Example 4) without a PCBM film, manufactured by applyingthe P3HT:PCBM blend layer and then performing thermal annealing at 90°C., had a power conversion efficiency of about 1.18%, whereas the deviceof Comparative Example 1 which included the PCBM film, had a powerconversion efficiency of 1.73%. Based on this, the power conversionefficiency was increased by about 50% (FIG. 4). Furthermore, the powerconversion efficiency of the device of Comparative Example 1 was similarto the power conversion efficiency of 1.74% for the conventional device(Comparative Example 6) subjected to thermal annealing at 150° C. (FIG.6). Accordingly, excellent power conversion efficiency can be obtainedat annealing temperatures much lower than those used for conventionalsolar cells. Thus a polymer solar cell using a plastic substrate havingthermal deformability can be advantageously manufactured.

As described herein, the present invention provides a polymer solar celland a method of manufacturing the same. According to the presentinvention, the polymer solar cell has an electron accepting layer, and aPCBM film layer formed between a blend layer of conductive polymer andelectron acceptor and, a second electrode, thereby improving theinterfacial properties of the second electrode and the electronacceptor. Thus, compared to conventional polymer solar cells, the powerconversion efficiency of the solar cell of the invention can beincreased by about 50% or more, and in particular, high power conversionefficiency can be attained even in a low-temperature thermal annealingprocess.

Although the preferred embodiments of the present invention have beendisclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed and claimed in the accompanying claims.

1. A polymer solar cell, comprising: a substrate; and a first electrodecomprising a conductive layer positioned on the substrate; a holeinjection layer positioned on the first electrode; a photoactive layerpositioned on the hole injection layer; an electron-accepting layerpositioned on the photoactive layer; a buffer layer positioned on theelectron-accepting layer; and a second electrode, wherein thephotoactive layer is a blend layer comprising a conductive polymer andan electron acceptor, and wherein an electron-accepting layer comprisesa C60-C70 fullerene derivative; and wherein the buffer layer comprisesLiF.
 2. The polymer solar cell of claim 1, wherein theelectron-accepting layer further comprises PCBM ([6,6]-phenyl-C₆₁butyric acid methyl ester).
 3. The polymer solar cell of claim 1,wherein the electron-accepting layer has a thickness of about 0.1 toabout 10 nm.
 4. The polymer solar cell of claim 1, wherein the holeinjection layer comprises material selected from the group consisting ofPEDOT-PSS (poly(3,4-ethylenedioxythiophene))-(poly(styrenesulfonate)),polyaniline-CSA, pentacene, Cu-PC (copper phthalocyanine), P3HT(poly(3-hexylthiophene), polysiloxane carbazole, polyaniline,polyethylene oxide, (poly(1-methoxy-4-(O-disperse Red1)-2,5-phenylene-vinylene), polyindole, polycarbazole, polypyridiazine,polyisothianaphthalene, polyphenylene sulfide, polyvinylpyridine,polythiophene, polyfluorene, polypyridine, and derivatives thereof, and;a combination comprising at least one of the foregoing.
 5. The polymersolar cell of claim 4, wherein the hole injection layer has a thicknessof about 5 to about 2000 nm.
 6. The polymer solar cell of claim 1,wherein the conductive polymer is one or more selected from a groupconsisting of P3HT (poly(3-hexylthiophene), polysiloxane carbazole,polyaniline, polyethylene oxide, (poly(1-methoxy-4-(O-disperse Red1)-2,5-phenylene-vinylene), polyindole, polycarbazole, polypyridiazine,polyisothianaphthalene, polyphenylene sulfide, polyvinylpyridine,polythiophene, polyfluorene, polypyridine, and derivatives thereof, and;a combination comprising at least one of the foregoing conductivepolymers.
 7. The polymer solar cell of claim 1, wherein the electronacceptor is selected from a group consisting of fullerene or derivativesthereof, nanocrystals, carbon nanotubes, carbon nanorods, carbonnanowires and; a combination comprising at least one of the foregoingelectron acceptors.
 8. The polymer solar cell of claim 6, wherein theconductive polymer is P3HT.
 9. The polymer solar cell of claim 7,wherein the electron acceptor is PCBM.
 10. The polymer solar cell ofclaim 1, wherein the conductive polymer and the electron acceptor aremixed at a weight ratio of about 1:0.8 to about 1:1.2.
 11. The polymersolar cell of claim 10, wherein the conductive polymer and the electronacceptor are mixed at a weight ratio of about 1:0.8.
 12. The polymersolar cell of claim 1, wherein the second electrode comprises metalsselected from the group consisting of, including magnesium, calcium,sodium, potassium, titanium, indium, yttrium, lithium, aluminum, silver,tin, lead, and; a combination comprising at least one of the foregoingmetals.
 13. The polymer solar cell of claim 1, wherein the secondelectrode is aluminum.
 14. A method of manufacturing a polymer solarcell, comprising: forming a hole injection layer on a substratecomprising a conductive layer; forming a photoactive layer on the holeinjection layer, wherein the photoactive layer is a blend layercomprising a conductive polymer and an electron acceptor; performingthermal annealing; forming an electron-accepting layer on thephotoactive layer; forming a buffer layer on the electron acceptinglayer; wherein the buffer layer comprises LiF; and forming a secondelectrode on the buffer layer.
 15. The method of claim 14, wherein thethermal annealing is performed at about 80 to about 110° C.